The present invention relates to organic light-emitting devices (xe2x80x9cOLEDsxe2x80x9d), and more specifically, to multi-color organic light-emitting devices, having a polymeric hole transporting layer, and exciton blocking and electron transport layers comprising small-molecule materials to provide low leakage and high efficiency. The OLEDs of the present invention are suitable for use in single color, multi-color and full-color, passive or active matrix displays.
Organic light-emitting devices (OLEDs) are a promising technology for next generation flat panel displays. OLEDs generally consists of small molecule or polymer layers disposed between an anode and a cathode. The polymer can by either self-emissive or doped with dye molecules. Upon application of an electric current, electrons and holes are injected from respective electrodes and recombine to form excitons that radiatively decay and emit light. The color of the light emission is determined by the energy levels of the emissive species. A functioning full-color OLED display requires the individual RGB pixels to emit the correct color, have low leakage current and have high yields. One of the major challenges in fabricating a full-color OLED display is the deposition of RGB OLED pixels on the same substrate, as conventional lithography processes employed in the semiconductor industry destroy the organic materials. Thermal vacuum deposition of small molecule materials through a shadow mask is the standard practice used in industry, but it suffers from poor material usage, low throughput, and cannot be easily scaled to large substrates. There are two promising patterning methods for polymer OLEDs that are particularly suited to high-speed, large-area manufacture: inkjet deposition of the emissive polymer or dopants and thermal dye transfer (T. R. Hebner and J. C. Sturm, Appl. Phys. Lett. 73 (13), 1913 (1999); T. R. Hebner et al., Appl. Phys. Lett. 72, 519 (1998); J. Bharathan et al., Appl. Phys. Lett. 72, 2660 (1998); K. Tada and M. Onoda, Jpn. J. Appl. Phys. 38, L1143 (1999); F. Pschenitzka and J. C. Sturm, Appl. Phys. Lett. 74 (13), 1913 (1999)). However, these pattern deposited polymer OLEDs suffers from low efficiency and high leakage current, due to the topology of the resultant films and the additional processing required. One reason for the low efficiency in these OLEDs is the high hole mobility in comparison to the electron mobility in the polymer layer. As a result, many holes traverse the entire device without recombining with an electron to form excitons. Or the excitons may be formed close to the metal cathode and quenched by the cathode before it can emit light.
A conventional OLED having a single polymer layer is shown in FIG. 1. A glass substrate 10 is first provided, forming the bottom layer of the device. Then, a transparent Indium Tin Oxide (xe2x80x9cITOxe2x80x9d) anode 15 is deposited on the glass substrate 10. An organic polymer layer 20 is deposited on the ITO anode 15, and a cathode 140 is deposited on polymer 20. As shown in FIG. 2a, the dye dopants can be selectively deposited on to the polymer 20 to produce sub-pixel elements, such as red, green, and blue light-emitting elements. Light generated by the device is emitted through the glass substrate 10 and anode 15, as indicated by the arrows A. Such devices, however, suffer from high current leakage and reduced efficiency. As shown in FIG. 2b, it is also known in the art to produce OLEDs consisting of a polymeric HTL 20 and a small-molecule ETL 35, typically Alq. However, to our knowledge, the light emission in all these devices is from Alq, which is green, thus separate RGB pixels cannot be achieved.
What would be advantageous, but has not yet been provided, is a multi-color OLED array having exciton blocking and electron transport layers to provide low leakage and high efficiency. It is accordingly an object of this invention to improve the efficiency and reduce the leakage current of OLEDs with pattern deposited polymer layers by depositing a small molecule blocking layer such as disclosed in U.S. patent application Pub. Nos. 2001/0,043,044 and 2001/0,052,751 on the polymer layer.
The present invention is directed toward combining the potential manufacturing features and advantages of pattern depositing polymeric materials together with being able to deposit one or more additional layers of a small molecule material over the polymeric layers so as to fabricate OLEDs having improved external quantum efficiencies. Such a method allows use of substantially any method know in the art for pattern depositing the emissive polymeric regions in a multicolor array of pixels. Furthermore, the polymeric layers of the OLED may use substantially any suitable combination of polymeric OLED structures known in the art, for example, an un-doped emissive polymer layer as the only polymer layer, a doped emissive polymer layer as the only polymer layer, an un-doped emissive polymer layer on top of a hole injection polymer layer, or a doped emissive polymer layer on top of the hole injection polymer layer.
The devices of the present invention are suitable for use in single color, multi-color and full-color, passive or active matrix OLED displays.
In one embodiment, the present invention provides full-color device comprising an array of RGB pixels. In this embodiment, the polymer layer is patterned with locally doped emissive dopants to provide discrete red, green and blue light-emitting subpixels.
In another embodiment, the invention provides a method of fabricating a display comprising an array of OLED structures, the method comprising the steps of preparing a substrate coated with an anode layer, depositing a polymeric layer over the anode layer, pattern depositing an array of emissive dopants onto the polymeric layer; depositing a blocking layer over the array of emissive dopants, depositing an electron transporting layer over the blocking layer, and depositing a cathode layer. The method of the invention may include the deposition of additional layers as provided herein.
The present invention also relates to a method of manufacturing a multi-color OLED having exciton blocking and electron transport layers. In one embodiment, a glass substrate is provided, on top of which an Indium-Tin Oxide (ITO) anode is deposited. An organic polymer layer is formed on the ITO anode, and emissive dopants are introduced into the organic layer to provide discrete, color-emitting regions capable of emitting various colors (i.e., red, green, and blue colors). These emissive dopants may be put down by ink jet printing in a single pass. A blocking layer is formed on the multi-color organic layer, on top of which an electron transport layer is deposited. Cathodes are then deposited on the electron transport layer to form a complete device.
It is an object of the present invention to provide a multi-color OLED array having exciton blocking and electron transport layers to provide a high-efficiency device having low leakage.
It is another object of the present invention to provide a multi-color OLED display having reduced reverse leakage current.
It is still another object of the present invention to provide a multi-color OLED display that prevents cathode-quenching of excitons generated by the OLED.
It is still another object of the present invention to provide a method of manufacturing a multi-color OLED display having blocking and electron transport layers.